Surface Pro 6 - Microphone stopped working in Skype for Business

Hi,

i have a very annoying Problem with a Surface Pro 6 losing the Microphone during a Skype session with Plantronics Device (Callisto 610-m)!

The Surface works fine and has all latest updates installed.

The Surface uses a Surface Dock Connector with latest Firmware (verified 10.09.19).

The users initiates a SfB session and can make a conversation for about 10-20 seconds.

Then Skype says that the Microphone stopped working.
After some attempts (disable/enable Mic) it´s working for a few minutes.

The Surface Pro 6 uses  Windows 10 Enterprise (18362, latest updates) and Skype for Business with a Plantronics Device.

We use a lot of Plantronics Calisto 610-M Skype for Business devices in our Company on differnent Client. These Clients are Dell Optiplex, Dell Precsion and Lenovo Laptops.

I already switched the 610-M with other devices in house, the error is still the same.

The 610-M is connected to the Surface Dock.

The user still hears the interlocutor on the 610-M Speaker, but microphone stopped.

Has anyone an idea?

thanks in Advance 

Jürgen